Answer: c Clarification: The process of generation of an image from a hologram is called reconstruction. In the process, a wave called reconstruction wave illuminates the hologram, thus giving rise to the desired image. Which of the following is used for the formation of holograms? Answer: d Clarification: Laser is highly coherent. Due to this, they are widely used in the reconstruction process.
In a hologram, each point contains light from the whole of the original scene. It is not possible to break a hologram in small pieces. Answer: b Clarification: Holograms can be broken into smaller pieces and they can be reconstructed to form the entire object. However, the size of the hologram is reduced, as the resolution decreases.
The image starts to get blurred. Because of the symmetry of repeated FFTs, this operation should then lead to a distribution spectral power that approximates the aperture function. To generate a power spectrum plot from a circular 2D spectral distribution, the 2D matrix was mapped to a polar coordinate system with the average power measured across all polar coordinates for each radius.
As this mapping is a poor one, the resultant curves appear quite noisy. These curves are shown together with the simulated power spectra in FIG. It can be seen that the best fit is obtained for the smallest aperture sizes; the reason for this is discussed below. Further tests to determine whether the intensity pattern seen is indeed that due to coherent noise include measuring the speckle contrast and the population statistics of the intensity patterns.
In theory, the ratio of the standard deviation to the mean intensity should be unity for a true speckle pattern. The variation in the pixel value histograms are as shown in FIG. Once again these show that the fit with the model is most accurate for the case of the smallest aperture size. The reason for the increasing deviation of the simulations from the ideal speckle characteristics with aperture size can be explained in terms of the aperture PSF.
In order to be truly simulating speckle, we require that each pixel in the image plane is the result of contributions from multiple pixels in the object plane. As the image plane can be described as the convolution of the image with the PSF of the aperture, the PSF must be sufficiently wide to contain multiple pixels within the PSF area.
From FIG. The alternative is to keep the simulation resolution the same and reduce the physical size of the screen by a factor of four in each dimension 0. However, in this case, after applying the aperture we rely on increasingly fewer data points in the simulation to construct the image plane from, leading to errors. This can be seen be reducing the aperture sizes by a factor of 4 and maintaining the same resolution FIG. By increasing the resolution of the simulation by the same factor that the aperture was decreased so that the number of pixels used to compose the image plane remains constant returns the intensity map to speckle statistics FIG.
These values show that at a higher resolution, the simulation produces exponential decaying aberrations with contrast values around 0. Calculating the spectral power distributions for the intensity field using these simulation parameters shows how close the model fits with theoretical predictions FIG. We now consider invariance with ocular aberrations: The statistical properties of laser speckle patterns do not change with the introduction of aberrations into the imaging system.
If the intensity patterns predicted by the model are caused by coherent noise then the graphs shown above should be the same with the introduction of aberrations in the pupil plane.
Aberrations of varying powers were shown not to affect the statistical properties of the intensity patterns formed in the image plane. Sample statistics taken from these aberrations are shown in FIG. We now describe some experimental results: FIG. However, the brightness penalty to achieve this level of speckle reduction is undesirably high.
A similar effect can be achieved without such a high reduction in brightness using a piezo-electric actuator and a binary phase diffuser, as shown in FIG. In summary, speckle can be accurately simulated when multiple points in the object contribute to each point in the image plane i. From the range of aperture sizes modelled it can be seen from FIG. Furthermore, the moving diffuser can be seen to significantly reduce the spectral power spectrum of a speckle field measured experimentally at lower spatial frequencies.
However, using too coarse a diffuser scatters light outside the collection angle of the final lens, significantly decreasing the brightness. Using a pixellated binary phase diffuser scatters light inside the collection cone of the final projection lens.
This range is then a sufficiently small to allow piezo actuation. The appearance of speckle in the final image is then decreased to a level which is tolerable to the viewer. A holographically generated intermediate image is formed at a Fourier transform plane, at which a piezo-electrically driven pixellated diffuser is located.
The diffuser is linked by an arm shown schematically to a piezo-electric actuator , coupled to a driver In this example system a waveplate 34 is employed to rotate the polarisation of the incident beam for the beamsplitter. A holographically generated intermediate image is formed at the Fourier transform plane of the demagnifying optics, at which a piezoelectrically driven pixellated diffuser is located. Again the diffuser is linked by an arm shown schematically to a piezo-electric actuator , coupled to a driver Optionally in this and the previously described arrangement an aperture may also be included in this plane to block off one or more of zero order undiffracted light , the conjugate image, and higher diffraction orders.
The reflector is implemented using dichroic filters for the blue and red wavelengths. The diffuser is linked to piezo-electric actuator by an arm Temporal Reduction of Speckle, reducing the power of the speckle spectrum within a finite spatial frequency bandwidth.
As previously mentioned, OSPR reduces the appearance of speckle by randomising the phase of each pixel in the projected subframe image. The higher the rate at which the subframes are projected, the lower the power of the speckle spectrum within a spatial frequency bandwidth determined by the pixel pitch of the holographic image.
Using a diffuser in the intermediate image plane allows the rate at which the phase changes over the scale of a diffuser pixel to be increased beyond that achievable by the microdisplay alone. This effect could be achieved by increasing the subframe rate of the microdisplay, but this would use additional processing power.
Spatial reduction of speckle, increasing the bandwidth over which power in the speckle spectrum can be reduced. When the diffuser is placed in the plane of the holographic image i. Preferably the diffuser is substantially transparent, so that substantially only the phase of the projected image is affected by the diffuser.
Without a diffuser, the phase within a pixel of the projected image generated using OPSR is uniform at a given instant in time but varies randomly over time. Reducing the pixel pitch of the diffuser below that of the holographic image acts to reduce the area in the projected image over which the phase is uniform at a given instant in time. Over the integration time of the eye, regions in the projected image which have phases that vary randomly with respect to each other will produce multiple speckle patterns that will average out.
To the eye it then appears as though these regions are incoherent with respect to each other. Moving the diffuser rapidly generates random phases on a scale that is smaller than the projected image pixel. This effect could additionally or alternatively be achieved by increasing the number of pixels of the microdisplay i. Embodiments of the technique are implemented in a system which generates two-dimensional images holographically. This, inter alia, relaxes the time constraint on the diffuser.
This substantially facilitates the use of a piezo-actuated diffuser. In some preferred implementations a bending piezoelectric actuator is employed, in embodiments coupled to an arm holding the diffuser. Further, by attaching 2 piezo benders at right angles it is possible to achieve movement of the diffuser in two dimensions, preferably in two substantially orthogonal directions. In embodiments the frequency of the diffuser movement may be such that the period is less than 1 sub-frame interval.
However the effect appears to saturate at high speeds. We now describe a first example process for designing and constructing the diffuser. The diffuser was generated using a photo-lithography process exposing, developing and etching a photoresist pattern on glass. This helps to avoids light being scattered outside the final projections lens, increasing displayed image intensity, and reduces other artefacts caused by larger feature sizes.
By contrast with a ground glass diffuser, a binary phase, pixellated diffuser has a predictable spatial frequency structure and hence a predictable cone of angles over which light is scattered. By adjusting the pixel pitch of the binary phase diffuser, the range of angles over which the light is scattered can be closely controlled. This is useful for finding a good balance between reduced speckle contrast and maximising both image brightness and projector throw angle. In other arrangements, however, the optimal pixellated, quantised phase diffuser is a specially-computed pattern which may appear random to the eye but in fact offers improved performance in speckle reduction and throughput.
However this distribution is not optimal for speckle reduction and one can generally do better than this to achieve improved speckle reduction. One preferred option is for the diffuser to be computed as a phase hologram which diffracts light into a uniform disc of a chosen angle, selected to fill preferably substantially exactly the clear aperture of the projection lens imaging the diffuser. This gives the optimal speckle reduction for a given clear aperture, but does not maximise depth of field.
SPIE Vol. The skilled person will understand that applications for the techniques we have described are not limited to holographic image display systems displaying images on a planar or curved 2D screen but may also be employed when displaying or projecting an image or pattern on any surface using coherent light, in, particular holographically. Applications for the described techniques we have described include in particular but are not limited to the following: mobile phone; PDA; laptop; digital camera; digital video camera; games console; in-car cinema; navigation systems in-car or personal e.
No doubt many effective alternatives will occur to the skilled person and it will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.
A holographic image display system for displaying an image holographically on a display surface, the system comprising: a spatial light modulator SLM to display a hologram;. A holographic image display system as claimed in claim 1 wherein said diffuser comprises a pixellated, quantised phase diffuser. A holographic image display system as claimed in claim 2 wherein a said pixel of said diffuser has one of a plurality of quantised phase levels, and wherein said quantised phase levels are computed to provide predetermined intensity distribution for light diffused by said diffuser.
A holographic image display system as claimed in claim 1 wherein said diffuser comprises a phase hologram. A holographic image display system as claimed in claim 5 wherein said phase hologram is configured to diffuse light onto a disc. A holographic image display system as claimed in claim 6 wherein said disc is substantially uniform.
A holographic image display system as claimed in claim 1 wherein said diffuser configured to diffuse light onto a disc, wherein said projection optics comprises an imaging device to image said diffuser, and wherein said disc at least substantially fills a clear aperture of said imaging device imaging said diffuser. A holographic image display system as claimed in claim 2 wherein said activator comprises a piezoelectric activator, and further comprising a driver for said piezoelectric activator, and wherein, in operation, said driver is configured to move said diffuser a distance of at least twice said diffuser pixel pitch, preferably at least five times said diffuser pixel pitch.
A holographic image display system as claimed in claim 1 wherein said system is configured to move said diffuser sufficiently quickly for a changing speckle pattern in an image displayed by the system, resulting from a changing random phase pattern imposed by said moving diffuser on said pixels of said intermediate image, to be integrated in the eye of a human observer to reduce a perceived level of speckle in an image displayed by the system.
A holographic image display system as claimed in claim 1 further comprising a processor having an input to receive image data for display and having an output for driving said SLM, wherein said processor is configured to process said image data to generate hologram data for said hologram displayed on said SLM, wherein said pixels of said intermediate image formed by said displayed hologram have an intermediate image pixel pitch, and wherein said diffuser has pixels with a diffuser pixel pitch smaller than said intermediate image pixel pitch.
A holographic image display system as claimed in claim 13 wherein said processor is configured to generate a plurality of temporal holographic subframes for display in rapid succession on said SLM such that corresponding temporal subframe images on said display surface average in an observer's eye to give the impression of said displayed image.
A holographic image display system as claimed in claim 1 wherein said actuator is configured to move said diffuser in two dimensions. A method as claimed in claim 16 wherein said diffuser comprises a pixellated, quantised phase diffuser with pixels having a pitch less than a pitch of pixels of said displayed image at said intermediate image surface, such that said speckle is reduced at a spatial frequency higher that a maximum spatial frequency of said displayed image.
A method as claimed in claim 16 wherein said diffuser has a pattern computed to substantially match an acceptance angle of an imaging device imaging said intermediate imaging surface of said projection optics. A method as claimed in claim 16 wherein said diffuser has a pattern computed to substantially maximise speckle reduction for a determined depth of field of said holographic image display system. A method as claimed in claim 16 wherein said moving comprises moving said diffuser in two dimensions.
An image display system to project an image onto a display surface using at least partially coherent light, the system comprising: a spatial light modulator SLM to display a two-dimensional image;.
An image display system as claimed in claim 21 wherein said intermediate image has an intermediate image pixel pitch, and wherein said diffuser has a pixel pitch less than said intermediate image pixel pitch. An image display system as claimed in claim 21 wherein said actuator is configured to move said diffuser in two dimensions.
An image display system as claimed in claim 21 wherein a said pixel of said diffuser has one of a plurality of quantised phase levels and wherein said quantised phase levels are computed to provide predetermined intensity distribution for light diffused by said diffuser.
An image display system as claimed in claim 21 wherein said diffuser comprises a phase hologram. An image display system as claimed in claim 25 wherein said phase hologram is configured to diffuse light onto a disc, wherein said disc is substantially uniform, wherein said projection optics comprises an imaging device to image said diffuser, and wherein said disc at least substantially fills a clear aperture of said imaging device imaging said diffuser.
USA1 en. EPA1 en. JPA en. GBB en. TWA en. WOA1 en. WOA2 en. USB2 en. System for tomographic imaging using coherent light that has a random phase distribution. USB1 en. Systems and methods for fabricating variable digital optical images using generic optical matrices.
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Previous image Next image. Despite years of hype, virtual reality headsets have yet to topple TV or computer screens as the go-to devices for video viewing.
One reason: VR can make users feel sick. Nausea and eye strain can result because VR creates an illusion of 3D viewing although the user is in fact staring at a fixed-distance 2D display. The solution for better 3D visualization could lie in a year-old technology remade for the digital world: holograms.
Holograms deliver an exceptional representation of 3D world around us. Go ahead — check out the holographic dove on your Visa card. Researchers have long sought to make computer-generated holograms, but the process has traditionally required a supercomputer to churn through physics simulations, which is time-consuming and can yield less-than-photorealistic results. Now, MIT researchers have developed a new way to produce holograms almost instantly — and the deep learning-based method is so efficient that it can run on a laptop in the blink of an eye, the researchers say.
The advance could fuel a spillover of holography into fields like VR and 3D printing. Shi worked on the study, published today in Nature , with his advisor and co-author Wojciech Matusik.
In contrast, a hologram encodes both the brightness and phase of each light wave. But despite their realism, holograms are a challenge to make and share. First developed in the mids, early holograms were recorded optically. And they were hard copy only, making them difficult to reproduce and share. Computer-generated holography sidesteps these challenges by simulating the optical setup. But the process can be a computational slog. They used deep learning to accelerate computer-generated holography, allowing for real-time hologram generation.
The team designed a convolutional neural network — a processing technique that uses a chain of trainable tensors to roughly mimic how humans process visual information.
The team built a custom database of 4, pairs of computer-generated images.
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